The present invention relates to a method for manufacturing silicon wafers from a silicon block whose lateral surfaces are ground before separation is carried out.
Silicon wafers are thin sheets of crystalline silicon and are used to manufacture solar cells and, therefore, solar modules. In accordance with the increasing usage of solar cells and the like, the demand for silicon wafers is also increasing from year to year.
The production of silicon wafers begins with the manufacture of blocks of crystalline silicon ingots that are made of monocrystalline and polycrystalline material and are cultivated in a round shape or as cuboids. In a further step, these crystalline ingots are separated or sawed to form further cuboid blocks. These blocks are also referred to as columns or bricks; in the case of monocrystalline silicon, the cross section is nearly square in shape, and in the case of multicrystalline silicon, the cross section is exactly square in shape. In a further step, these blocks are sawed into the individual wafers, and, in fact, usually transversely to their longitudinal direction, so that the lateral surfaces of the column-shaped blocks or bricks created via sawing become the edge surfaces of the wafers produced in this manner. It has been shown that, when the bricks are manufactured, superfine, often microscopically small fissures are produced on the lateral surfaces created via sawing. These fissures penetrate the block, and therefore, the edges of the subsequent wafer, to a greater or lesser extent.
Silicon is an extremely brittle material. Unlike the situation with metals, a fissure in this material may therefore spread rapidly when stress is applied. Normal stressing, which occurs, e.g., when the cells are handled in production, may quickly result in fracture of the wafer if fissures are present, in particular in the edge of the wafer.
Since the costs required to manufacture the silicon wafer comprise approximately 55% of the total manufacturing costs for solar cells, a high percentage of broken silicon wafers, i.e., a high fracture rate, results in a marked increase in costs to manufacture solar cells.
Given that there is a worldwide shortage of silicon, the silicon wafers manufactured in the future will have to be extremely thin. If fissures form in these thin silicon wafers, they are extremely susceptible to fracture. Fissures in the edges of the wafers are particularly critical.
To avoid the problems described above, it is provided in US 2002 036182 AA to diminish the roughness of the surfaces of such silicon blocks before the silicon wafers are manufactured. The roughness of the surfaces is diminished via mechanical polishing, as described in JP 3,648,239. In the best case described (example 6), the roughness Ry attained is only less than 1 μm. This is not nearly adequate for the manufacture of wafers, in particular those with a thickness that is less than or equal to 180 μm. In addition, the method variants described—“processing with loose grain” (method 1 and sponge wheel), processing with brushes with and without adhesive abrasive grains on the brush hairs, and processing with abrasive grains embedded in a sponge wheel—are definitely not suitable for the purpose of leaving defined, direction-critical processing tracks, which are necessary in order to obtain a greatly improved surface roughness on the lateral surfaces of the silicon bricks. Finally, a separating method for a semiconductor is described in U.S. Pat. No. 5,484,326, with which the surfaces of a silicon block are ground in advance.
It has been shown that this method does reduce the frequency of fracture of the cut silicon wafers, but that this is insufficient for very thin wafers in particular.